Previously known endodontic root canal files have been primarily made from carbon steel or stainless steel wire blanks which are ground to a desired size, taper and cross-sectional shape (for example, square, triangular or rhomboid). The wire blank is gripped on a first end while spring-loaded jaws secure the ground portion of the blank. The blank is then rotated from the gripped end while the jaws are moved axially away from that end. The jaws which secure the ground portion move along the ground wire blank, but do not allow the distal end of the blank to twist, thereby forming a twisted portion and helical flutes from the edges of the blank between the gripped end and the jaws. The cross-sectional shape, size and taper as well as the speed of twisting and spring force may be controlled to attain the desired properties in the final product. One such endodontic instrument is shown in U.S. Pat. No. 4,443,193, herein incorporated in its entirety by reference.
With the introduction of superelastic materials such as nickel titanium alloys, it has been recognized that superelastic endodontic files would provide more elastic flexibility in bending and torsion than the previous steel files. The paper, An Initial Investigation of the Bending and Torsional Properties of Nitinol Root Canal Files, Walia et al., Journal of Endodontics, Volume XIV, No. 7, July 1988, studied the feasibility of manufacturing superelastic nickel titanium root canal files and evaluated the bending and torsional properties of these instruments. In order to fabricate nickel titanium files the Walia article discloses the machining of the fluted structure of a K-type file directly on cylindrical wire blanks.
U.S. Pat. Nos. 5,464,362, 5,527,205 and 5,628,674 disclose the machining process used to grind the files disclosed in the Walia et al. article in which the fluted structure of the K-type file is ground directly on a cylindrical nickel titanium wire blank to create cutting edges at the apices. U.S. Pat. No. 5,464,362 provides a cylindrical rod of superelastic material having a diameter between 0.0024" and 0.062" which is translated axially, relative to a rotating grinding wheel, at an axial feed rate of between about 3 inches per minute and 8 inches per minute, with the preferred rate being not more than about 5 inches per minute. The rod is rotated about its longitudinal axis while it is translated relative to the wheel so that the wheel removes at least about 25% of the diameter of the rod at the point of maximum removal and forms at least one helical flute in the rod. In order to accomplish this grinding, the wheel is rotated at a surface speed of not more than about 3,000 surface feet per minute and preferably not more than about 2,200 surface feet per minute. The grinding wheel also has a grit size greater than about 200 grit and preferably greater than about 220 grit. If the file of the U.S. Pat. No. 5,464,362 patent is to have more than one helical flute, a first flute is ground, the rod is indexed about its longitudinal axis either by 180.degree. (two flutes), or 120.degree. (three flutes), and the grinding step is repeated.